Customize: Introduce starter content and site freshness state.

A theme can opt-in for tailored starter content to apply to the customizer when previewing the theme on a fresh install, when `fresh_site` is at its initial `1` value. Starter content is staged in the customizer and does not go live unless the changes are published. Initial starter content is added to Twenty Seventeen.

* The `fresh_site` flag is cleared when a published post or page is saved, when widgets are modified, or when the customizer state is saved.
* Starter content is registered via `starter-content` theme support, where the argument is an array containing `widgets`, `posts`, `nav_menus`, `options`, and `theme_mods`. Posts/pages in starter content are created with the `auto-draft` status, re-using the page/post stubs feature added to nav menus and the static front page controls.
* A `get_theme_starter_content` filter allows for plugins to extend a theme's starter content.
* Starter content in themes can/should re-use existing starter content items in core by using named placeholders.
* Import theme starter content into customized state when fresh site.
* Prevent original_title differences from causing refreshes if title is present.
* Ensure nav menu item url is set according to object when previewing.
* Make sure initial saved state is false if there are dirty settings without an existing changeset.
* Ensure dirty settings are cleaned upon changeset publishing.

Props helen, westonruter, ocean90.
Fixes #38114, #38533.


git-svn-id: https://develop.svn.wordpress.org/trunk@38991 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/schema.php

@@ -516,6 +516,9 @@ function populate_options() {
 	// 4.4.0
 	'medium_large_size_w' => 768,
 	'medium_large_size_h' => 0,
+
+	// 4.7.0
+	'fresh_site' => 1,
 	);
 
 	// 3.3